I intend to select Chinese Hamster Ovary (CHO) cells that are resistant to picornavirus infection at restrictive temperatures. These cells will be selected by two methods. One method will identify mutagenized cells that survive a picornavirus infection that normally would be lethal. The other method will identify cells which allow an abortive yet lethal infective process. The resistant cells will then be screened for 1) their ability to grow picornavirus (encephalomyocarditis virus, (EMC)) at permissive temperature. Those cells that will not grow virus at restrictive temperature but do adsorb the virus will be further tested for their ability to grow normally at restrictive temperature in the absence of virus. This final criterion will divide the defective cells into two classes, a) those that have a defect in an essential host process and b) those that have a defect in a non-essential host process. The defects in these cells will then be characterized by well established techniques. Such cells will have considerable significance. Initially, they will allow the direct investigation of host participation in picornavirus infection. Subsequently, they will allow the characterization of cell conditional-lethal defects that otherwise would be inaccessible to investigation.